This invention concerns a system for detoxifying chemical wastes. More particularly, this invention concerns a system for detoxifying the discharges from electroplating, metal finishing, and other chemical processes involving heavy metal salts, including the recovery of metals therefrom, as well as a portable resin cartridge for use in the system.
Over the past decade and a half, there has been a growing awareness in government, industry, and the general public alike that more sophisticated methods must be used in the management of the discharges of our industrial society or our health and well being will suffer significantly. This awareness has resulted in legislation and concomitant regulations which often address the problem in terms of prohibitions, with the result that the atmosphere in which efforts toward adequate detoxification or reprocessing for reuse of materials have been attempted has been clouded by confusion, fear, and accusation.
A major social aspect of this particular problem is that the metal finishing industry is, to a large degree, made up of a number of independent job shops clustered in metropolitan areas. These job shops are usually moderate in size and lack sophisticated in-house technical staff. They are also mostly sole proprietorships which, if faced with economically impractical alternatives, would shut down, thus depriving their home communities of employment opportunities and tax revenues.
The United States Environmental Protection Agency (hereafter called EPA) recommended treatment of electroplating wastes consists of reduction of hexavalent chromium to trivalent chromium and oxidation of cyanide, followed by precipitation of all metals together as hydroxides. The high investment and operating costs for this system would force many electroplating job shops to shut down their operations. EPA estimates show that 21-36% of the metal finishing industry will be forced to close should compliance with regulations be enforced. The smaller operators would be the ones most affected. See "Economic Analysis of Effluent Guidelines--The Metal Finishing Industry," United States Environmental Protection Agency, EPA 230/1-74-032 (1974).
The regulatory aspects concerning this problem have been promulgated by the EPA in the Metal Finishing effluent limitations. See "Electroplating and Metal Finishing Point Source Categories, Effluent Limitations Guidelines, Pretreatment Standards and New Source Performance Standards," United States Environmental Protection Agency, Federal Register, July 15, 1983. This regulation sets concentration limits on the discharge of pollutants from electroplating and metal finishing facilities into U.S. waters or into publicly owned treatment works. The reference gives a summary of the regulation, background of the regulation, the legal authority, and the compliance schedule. The compliance date for existing non-integrated electroplating plants was Apr. 27, 1984.
The major technical aspects of the problem derive from the need to process relatively large volumes of rinse water which contain only small amounts of dissolved metal salts and to significantly reduce the concentration of these already dilute solutions to meet discharge regulations. Further, it is necessary to process the rinse waters from each metal finishing line separately so as to maintain the segregation of the metals involved, if they are to be recovered for reuse or sale. The separation of mixed metals, except in special cases, is very difficult and expensive.
Several alternative treatments have been suggested in the literature to enable an electroplater to meet the regulations for discharge of metals and cyanide. These alternatives include the following methods:
(a) Concentration methods like evaporation, reverse osmosis and electrodialysis; PA1 (b) Separation methods like activated carbon adsorption and ion exchange; PA1 (c) Precipitation methods like the EPA recommended "destruct" system. PA1 (a) Reverse osmosis--This method uses a semi-permeable membrane which allows passage of pure water but not dissolved salts or other materials. The application of pressure to the contaminated water causes the pure water itself to cross the membrane in the direction opposite to the osmotic pressure, with the result that the dissolved materials become more concentrated and the pure water passing through the membrane is available for reuse. PA1 (b) Ion exchange--In this method the rinse water effluents are passed through beds of ion exchange resins which selectively remove the metal ions. The resins can be regenerated for subsequent reuse and the metal ions can be recovered from the regenerant solutions. PA1 (c) "Destruct" system--This involves the reduction of hexavalent chromium to trivalent chromium, the oxidation of the cyanide, and then the precipitation of all the metals as hydroxides by treatment with caustic or lime.
Detailed reviews of these methods have been presented elsewhere. See "Recovery of Metal Values from Electroplating Industry Wastewaters," by A. Gupta, Department of Chemical Engineering, Princeton University (1982). The more practicable of these alternatives are the following:
On-site recycling techniques like reverse osmosis and evaporation may be economically feasible if there is sufficient metal to be recovered. Separate units are required for each metal. In large plating installations the value of metal recovered might pay back the investment in such techniques. It would be difficult to recover this investment if the metal discharge rate is relatively low, as is the case for most job shops. On-site recovery also requires skilled operators trained in the specific techniques employed. This is often not economically feasible for job shops.
While the "destruct" precipitation system has the advantage that it can process mixed metal inputs in-plant, it has several major disadvantages, as follows:
(1) The precipitation and clarification equipment require a lot of space. It is estimated that nearly 5000 square feet will be needed for a typical 38-employee establishment. See "Economic Analysis of Effluent Guidelines--The Metal Finishing Industry," United States Environmental Protection Agency, EPA 230/1-74-032 (1974). This might be a problem with job shops, many of which have little free floor space. PA0 (2) Experienced and skilled personnel are required to operate the system properly. Metal hydroxide precipitates are difficult to filter or dewater. In addition, precipitation from a dilute solution is difficult to control. PA0 (3) The "destruct" precipitation system is typically operated as a continuous feed process and is easiest to control when the incoming concentrations of metals are constant. The electroplating processes linked to it are typically batch processes whose metal discharge concentrations vary widely. This is a significant materials flow mismatch. PA0 (4) The destruct system produces metal hydroxide sludges which have to be transported to a hazardous waste landfill. The sludge disposal costs have been rising and are expected to rise rapidly in the future. Regulations banning land disposal of such materials are in effect in some states and are expected in others. Even where land disposal is available, the generator of these wastes is liable for any future damage resulting from the leakage of the landfill. PA0 (5) No economical method is available to recover the metal values from these mixed metal hydroxide sludges, so the metals cannot be recycled. Since many metal sources are getting depleted, their recovery is important to our long term needs. PA0 (6) The high investment and operating costs for the "destruct" system would force many job shops to shut down their operations. As was indicated above, EPA estimates show that 21-36% of the metal finishing industry will be forced to close and that the smaller operators will be the most affected. Other estimates of job shop closures are considerably higher.
The idea of relocating the electroplaters in close proximity to each other and then using a joint "destruct" system for the accumulated waste waters had been envisaged. This would not be very practical because most of the job shops have been located at their current locations for a number of years and have been shown to be extremely unwilling to move their operations to a new site adjacent to their competitors (e.g., failure of the Plating City project, Brooklyn Economic Development Corporation, Brooklyn, N.Y.).
In contrast to the "destruct" system, ion exchange equipment is very compact, hence its space requirement is very low. The main advantage of ion exchange is that it is quite versatile. A batch process like electroplating can be integrated with ion exchange treatment easily. Variations in concentration of metal ions and changes in flow rate can be easily handled.
Another major advantage of ion exchange is that it is a recovery process. The need for landfilling is eliminated, as no sludge is generated. Valuable metals are recovered for reuse. It is the only recovery technology adaptable to centralized treatment. Centralized treatment offers considerable savings to small job shops because of the pooling of costly equipment and the economies of scale. A central facility can also operate more profitably because it has access to more resources and technical expertise than the individual electroplater.